1. Field of the Invention
The present invention is directed to a method and apparatus for producing a discrete or composite image on a CRT (cathode ray tube) screen from selected digital data stored in one or more memory storage areas.
2. Description of the Prior Art
Some graphics display terminals of the prior art utilized the digital data stored in a number of memory storage areas, hereinafter referred to as bit planes, to generate a display on a CRT. Referring to FIG. 1, the prior art terminal's raster memory comprises three bit planes. For each pixel location on the CRT screen, there is a one-bit memory cell in each of the bit planes. Referring to FIG. 2, for any one pixel, the bits in each bit plane collectively constitute a pixel word. The pixel word is used as an address to a table stored in a color map memory, and a corresponding brightness index is noted and read therefrom. The brightness index, a binary number, is converted into an analog signal via a D to A converter, the analog signal energizing the cathode ray tube to determine the brightness of the image at the particular pixel. FIG. 2 illustrates the hardware needed to generate a color display. In this figure, there are three D-A convertors, one for each of the additive primary colors. The outputs therefrom determine the brightness of the red, green, and blue phosphors for each pixel of the display.
To display information on the screen, circuitry in the terminal scans all of the bit planes, simultaneously. For each pixel on the screen, the circuitry reads one binary bit from each plane. All of the bits for each pixel location collectively form the pixel word mentioned hereinabove.
Since all of the bit planes are scanned simultaneously; and since all of the bits read therefrom collectively form the pixel word, only one image could be formed on the CRT. This limits the terminals capability. If it was desired to create two or more images and to display the images on the CRT either independently of one another or superimposed onto one another, the graphics display terminals of the prior art could not perform this function.